Medical system including a novel bipolar pacing pair

ABSTRACT

An implantable electrical medical system comprises a low voltage cathode electrode assembly, including a cathode surface adapted for, intimate contact with electrically active tissue, and a low voltage anode electrode assembly, including an anode surface and a porous layer extending over the anode surface. The cathode surface and the anode surface function as a bipolar pair for pacing and the porous layer extending over the anode surface allows conduction therethrough and prevents the anode surface from contacting the electrically active tissue in order to prevent anodal stimulation.

This application is a continuation-in-part of application Ser. No.10/439,201, filed May 15, 2003 now U.S. Pat. No. 7,191,016.

FIELD OF THE INVENTION

Embodiments of the present invention generally relate to the field ofcardiac pacing and/or defibrillation, and more particularly to enhancedleft heart pacing.

BACKGROUND

In the field of cardiac pacing and/or defibrillation, therapy deliveryfrom an implanted medical device may rely upon cardiac signals sensedand pacing therapy delivered via a bipolar pair of implanted electrodesincluded on one or more medical electrical leads coupled to the medicaldevice.

With respect to sensing, accurate detection and classification ofarrhythmias relies upon an adequate signal-to-noise ratio picked up bythe bipolar pair of electrodes; the signal being a near-field cardiacconduction signal and the noise being either a far-field cardiacconduction signal or electrical activity in other muscles of the body ora combination thereof. Many medical devices incorporate sensingalgorithms to blank or ignore far-field signals, however this may leadto under-sensing or under-detection of fast regular rhythms. As analternative, a spacing between the bipolar pair of electrodes on thelead may be decreased in order reduce and localize the field of sensingbetween the two electrodes.

With respect to pacing, an effective stimulating pulse is focused viaintimate tissue contact with a first electrode, serving as a cathode,included in the bipolar pair; if a second electrode of the bipolar pair,serving as an anode, comes too close to active tissue there is a chanceof anodal stimulation, which impairs the therapy delivery.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of theinvention and therefore do not limit the scope of the invention, but arepresented to assist in providing a proper understanding. The drawingsare not to scale (unless so stated) and are intended for use inconjunction with the explanations in the following detailed description.The present invention will hereinafter be described in conjunction withthe appended drawings, wherein like numerals and letters denote likeelements, and:

FIG. 1 is a schematic of a medical system according to one embodiment ofthe present invention;

FIG. 2A is a plan view of a medical electrical lead according to oneembodiment of the present invention;

FIG. 2B is a plan view of a distal portion of a medical electrical leadaccording to another embodiment of the present invention;

FIG. 2C is a plan view of a distal portion of a medical electrical leadaccording to yet another embodiment of the present invention;

FIGS. 3A-C are enlarged schematic plan views of portions of porouslayers according to alternate embodiments of the present invention;

FIG. 4A is a schematic of a lead system implanted within a right side ofa heart according to one embodiment of the present invention;

FIG. 4B is a series of signal traces illustrating function according toembodiments of the present invention;

FIG. 5 is a graph illustrating results of a twelve-week animal studyevaluating embodiments of the present invention; and

FIG. 6 is a schematic representation of a medical system according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 is a schematic of a medical system according to one embodiment ofthe present invention. FIG. 1 illustrates the medical system includingan implantable medical electrical lead 2 coupled an implantable medicaldevice (IMD) 1 via a connector header 15; connector header 15 includes abore 16 to receive a connector formed at a proximal end 21 of lead 2wherein electrical contacts 17 and 18 of header 15 couple with leadcontacts 22 and 23 of connector, respectively. Header 15 is attached toa hermetically sealed enclosure 10 that contains a battery, electroniccircuitry and other components known to those skilled in the art, andelectrical contacts 17 and 18 are any type known to those skilled in theart that are electrically connected via feedthroughs (not shown) mountedto extend through hermetically sealed enclosure 10 in order toelectrically couple lead 2 with IMD 1.

FIG. 1 further illustrates lead 2 including first electrode 26 joined toa lead body 30 in proximity to a distal end 24 and a second electrode 25joined to lead body 30 in proximity to first electrode 26 and spaced adistance X from first electrode 26. First electrode 26 and secondelectrode 25 are electrically coupled to lead contacts 22 and 23 viainsulated conductors (not shown) extending along lead body 30. Accordingto embodiments of the present invention, first electrode 26 and secondelectrode 25 form a bipolar pair, each having a surface area adapted forlow voltage pacing and sensing, and distance X between first electrode26 and second electrode 25 is less than approximately 9 millimeters;furthermore, first electrode 26 has a negative polarity and is adaptedfor intimate contact with tissue at an implant site and second electrode25 has a positive polarity and is prevented from having direct touchingcontact with tissue adjacent to the implant site by a porous layer(FIGS. 2-3) formed over second electrode 25. Electrodes 25 and 26, andother electrodes described herein, according to some embodiments, arecomprised of a platinum-iridium alloy. First electrode 26, and otherfirst electrodes described herein, may have a porous surface structureenhancing intimate tissue contact as well as a steroid coating formedthereover or a plug comprising steroid formed therein. Detailsassociated with electrode fabrication and alternate electrode materials,including, but not limited to titanium, tantalum, ruthenium, and carbon,are well known to those skilled in the art of lead construction. Itshould be noted that positions of first electrode 26 and secondelectrode 25 might be switched according to alternate embodiments of thepresent invention.

FIG. 2A is a plan view including a cut-away section of a medicalelectrical lead 20 according to one embodiment of the present invention,which may be coupled to an IMD just as lead 2 is coupled to IMD 1illustrated in FIG. 1. Examples of IMD's suitable for operation inaccordance with embodiments of the present invention include, but arenot limited to the following Medtronic products: GEM, Marquis DR, JewelAF, AT 500, and Kappa 900.

FIG. 2A illustrates a body 300 of lead 20 including a distal end 240 anda connector 50 formed at a proximal end 210; a first electrode 260,joined to distal end 240, is coupled to a first contact 220 of connector50 via a cable conductor 3 extending within insulating sheath 5, and asecond electrode 251, joined to lead body 300 in proximity to firstelectrode 260 and having a porous layer 252 formed thereover, is coupledto a second contact 230 via a coil conductor 6. Coupling of conductors 3and 6 to first electrode 260 and second electrode 251 and to firstcontact 220 and second contact 230 may be accomplished by means of weldsor crimps known to those skilled in the art; FIG. 2A illustrates cableconductor 3 coupled to first electrode 260 via a coupling component 4wherein cable may be crimped and electrode 260 may be welded.

According to embodiments of the present invention, a bipolar pair forpacing and sensing is formed by first electrode 260 functioning as acathode and second electrode 251 functioning as an anode; layer 252 oversecond electrode 251 allows conduction therethrough while preventingdirect touching contact of electrode 251 with tissue adjacent to animplant site, into which electrode 260 would be fixed. FIG. 2A furtherillustrates first electrode 260 formed as a helix for fixation to theimplant site, however, according to other embodiments a first electrodemay be formed around or within a fixation helix for example along asurface 262 circumscribing helix or on a surface 261 formed in center ofhelix wherein fixation of helix into an implant site will bring surfaces261, 262 into intimate contact with tissue at the implant site.According to one embodiment, second electrode 251 is recessed so that anouter surface 201 of layer 252 is isodiametric with an outer surface 200of lead body 300, as illustrated in FIG. 2A. FIG. 2B is a plan viewincluding a cut-away section of a distal portion of a lead 70illustrating an alternate embodiment wherein an outer surface 202 of aporous layer 254 formed over a second electrode 253 has a diametergreater than outer surface 200 of lead body 300.

FIG. 2C is a plan view of a distal portion of a lead 80 according to yetanother embodiment of the present invention. FIG. 2C illustrates a firstelectrode 860 formed as a generally hemispherical dome and a secondelectrode 853 (shown by dashed lines) joined to lead body 300 inproximity to first electrode 860 and having a porous layer 854 formedthereover. FIG. 2C further illustrates a tine structure 880 formed aboutfirst electrode 860 in order to maintain intimate contact of firstelectrode 860 with tissue at an implant site.

According to embodiments of the present invention a maximum thickness ofa porous layer covering a sensing anode, such as layers 252, 254, 854,is between approximately 0.005 inch and approximately 0.020 inch, andpore sizes of the layer, on average are between approximately 0.4 micronand approximately 50 microns. FIGS. 3A-C are enlarged schematic planviews of portions of various porous layers according to alternateembodiments of the present invention. FIGS. 3A and 3B illustrates pores91 and 92, respectively formed in a substantially uniform pattern, whileFIG. 3C illustrates pores 95 formed in a substantially random manner.According to one embodiment of the present invention a porous layer maybe formed of a polymer, such as silicone or polyurethane, wherein pores,e.g. 91, are holes formed by mechanical means, e.g. drilling, or thermalmeans, e.g. laser perforation, as generally illustrated in FIG. 3A.According to an alternate embodiment of the present invention, a porouslayer may be formed of a polymer material having a porous microstructuresuch as expanded polytetrafluoroethylene (e-PTFE), as is generallyillustrated in FIG. 3B wherein pores 92 are formed by a network offibrils 93 connected at nodes 94. Furthermore, an alternate embodimentof a porous layer may employ a sheet of collagen as illustrated in FIG.3C, wherein pores 95 are formed by a network of collagen fibers 96.FIGS. 3A-C present exemplary porous layers; according to the presentinvention any type of porous material, which is biocompatible andphysically separates a low voltage sensing anode (e.g. second electrodes251, 253, 853), of a bipolar pair, from tissue in proximity to animplant site while allowing adequate electrical conduction therethroughis in accordance with the spirit of the present invention. In a subsetof embodiments a porous layer includes pore sizes, on average, rangingbetween approximately 0.4 microns and approximately 20 microns in orderto prevent chronic tissue ingrowth. Some embodiments of the presentinvention wherein a porous layer is hydrophobic, e.g. e-PTFE, include awetting agent impregnated within or spread over the porous layer inorder to facilitate passage of fluid through the porous layer necessaryfor electrical conduction. Examples of wetting agents includesurfactants, hydrogels, gelatins or combinations thereof; the use of twosurfactants, sodium dioctyl sulfosuccinate (DSS) andtridodecylmethylammonium chloride (TDMAC), in conjunction with e-PTFE istaught by Carson in U.S. Pat. No. 5,931,862, the teachings from whichare incorporated herein. Alternate embodiments employ porous layers,surfaces of which are treated to enhance wettability; examples oftreatments include but are not limited to plasma processes.

FIG. 4A is a schematic of a lead system implanted within a right side ofa heart according to one embodiment of the present invention. FIG. 4Aillustrates an atrial lead 350 implanted within an atrium 60 by means ofa first electrode 356 fixed to an atrial appendage implant site 62, anda ventricular lead 7 implanted within a ventricle 65 with a tipelectrode 13 at an apical implant site 162. According to embodiments ofthe present invention, leads 350 and 7 are coupled to an IMD, such asany of the aforementioned exemplary IMD's, to form a dual chamber systemwherein a second electrode 355 of atrial lead 350, positioned in closeproximity to first electrode 356, for example spaced a distance X (FIGS.1 and 2) from first electrode 356, includes a porous layer formedthereover (FIGS. 2 and 3) to prevent direct touching contact withcardiac tissue along wall 63 adjacent to implant site 62. Firstelectrode 356 and second electrode 355 form a bipolar pair, firstelectrode 356 being a cathode and second electrode 355 being an anode,for pacing and sensing, wherein sensing of near field cardiac signals,or P-waves, is enhanced as illustrated in panel C of FIG. 4B. Accordingto embodiments of the present invention, enhanced sensing of near fieldsignals improves detection and classification of arrhythmias fordelivery of appropriate therapy via lead 350 and/or lead 7 from an IMD.In some embodiments according to the present invention, a secondelectrode 12 of lead 7 also includes a porous layer formed thereover forenhanced sensing of near-field signals within ventricle 65. As furtherillustrated in FIG. 4A, leads 350 and 7 may also include defibrillationelectrodes 29 and 14, respectively, shown with dashed lines, fordelivery of high voltage stimulation.

FIG. 4B is a series of signal traces, shown in three panels, A, B and C.In each panel, the top traces, A1, B1 and C1 represent near-fieldsignals sensed by a bipolar pair, while the bottom traces, A2, A3, B2,B3 and C2, C3 represent unipolar components of each top trace: A2, B2and C2 are signals from a first electrode in intimate contact withtissue at an implant site, and A3, B3, and C3 are signals from a secondelectrode spaced proximally from the first electrode. Signal trace A1illustrates sensing by a bipolar pair of electrodes spaced approximately9 millimeters apart, signal trace B1 illustrates sensing by a bipolarpair of electrodes spaced approximately 4 millimeters apart and signaltrace C1 illustrates sensing by a bipolar pair of electrodes spacedapproximately 4 millimeters apart wherein the second electrode includesa porous layer formed thereover to prevent direct touching contact withtissue adjacent to the implant site, for example second electrode 355illustrated in FIG. 4A, according to an embodiment of the presentinvention. As illustrated in FIG. 4B, signal trace C3 has a higheramplitude and slew rate than signal trace A3 but a lower amplitude andslew rate than signal trace B3, which is almost identical to B2;therefore, a combination of closer spacing between electrodes and aporous layer separating second electrode from direct touching contactwith tissue adjacent to an implant site results in the largestpeak-to-peak amplitude of the bipolar signal, C1, illustrated in FIG.4B. Furthermore, according to embodiments of the present invention,prevention of direct touching contact between an anode, for examplesecond electrodes 355 and 12 illustrated in FIG. 4A, and electricallyactive tissue via a porous layer prevents anodal stimulation of thetissue.

EXAMPLE

A first type of lead including a ring electrode (anode) having a porouslayer formed thereover and spaced 4 millimeters from a helical tipelectrode (cathode) was compared to a second type of lead including aring electrode spaced 9 millimeters from a helical tip electrode. Twotypes of porous layers were employed in the first type of lead used inour study: 1.) a layer of polyurethane having a thickness ofapproximately 0.008 inch and a durometer of approximately 80 on a shoreA scale, wherein holes, having on average a diameter of 0.001 inch, wereformed by an excimer laser; and 2.) a layer of e-PTFE, obtained fromZeus (part no. 2E055-010 EO*AC), having a thickness of approximately0.010 inch and including pores having, on average, a size betweenapproximately 10 microns and approximately 20 microns. Both leads wereimplanted in a right atrial appendage of six sheep for 12 weeks.Unfiltered P-wave and far-field R-wave (FFRW) amplitudes were measuredduring sinus rhythm (SR) at implant, and 1, 3, 5, 8, and 12 weeks underisoflurane anesthesia. Atrial fibrillation (AF) was induced with 50 Hzrapid pacing and vagal stimulation at 12 wks, at which time, bipolarelectrograms from both leads were input to an ICD atrial sense amplifier(band pass: 16 to 46 Hz), during the AF to evaluate sensing performance.

Previous studies have shown that reducing the tip-to-ring spacing (TRS)reduces FFRW oversensing; however, short TRS has been associated withreductions in P-wave amplitude due to a close proximity of the anode totissue adjacent to the implant site resulting in contact between theanode and active tissue. The results of our study indicate that a shortTRS is feasible when the anode does not contact electrically activetissue, being separated by a porous layer. FIG. 5 presents a graphillustrating our results in which no difference was found betweenchronic P-wave amplitudes sensed by the second type of lead having a 9mm TRS and the first type of lead having a 4 mm TRS, and wherein FFRWamplitudes were 50% lower as sensed by the first type of lead having the4 mm TRS. Furthermore, our study found no difference in pacingthresholds between the second type of lead having the 9 mm TRS and thefirst type of lead having the 4 mm TRS and the porous layer of e-PTFEformed over the anode. Finally, no difference in sensing performance wasfound between unfiltered and filtered signals obtained from both typesof leads compared at week 12.

FIG. 6 is a schematic representation of a medical system according toanother embodiment of the present invention, wherein a first electrode656, which forms a bipolar pair with a second electrode 612, is joinedto a separate lead 600. FIG. 6 illustrates an exemplary system employedfor dual chamber pacing including a first electrode 656 of a first lead600 and a second and third electrode 612, 613 of a second lead 670coupled to medical device 1 via lead contact 622 of first lead 600engaged by electrical contact 617 in a header bore 615 and lead contacts603 and 623 of second lead 670 engaged by electrical contacts 602 and618, respectively, in a header bore 616. FIG. 6 further illustratesfirst lead 600 implanted in a coronary vein 660 for stimulation andsensing of a heart left side 680 and second lead 670 implanted in rightventricle 65 for sensing and stimulation of right ventricle 65; such anarrangement of leads included in a medical system is well known to thoseskilled in the art cardiac resynchronization therapy. According to anembodiment of the present invention, second electrode 612 includes aporous layer formed thereover of any of the previously describedembodiments (FIGS. 2A-3C); the porous layer allows conductiontherethrough while preventing direct contact of second electrode 612with tissue along a wall 663 adjacent apical implant site 162 in orderto prevent anodal stimulation of right ventricle 65 when secondelectrode 612 forms a bipolar pair with first electrode 656 forstimulation of heart left side 680. Construction of first and secondleads 600, 670 is similar to that described in conjunction with FIGS.2A-C and materials and methods are well known to those skilled in theart. Furthermore, as previously described, first electrode 656 isadapted for intimate tissue contact and construction details for suchelectrodes are also well known to those skilled in the art.

While specific embodiments have been presented in the foregoing detaileddescription, it should be clear that a vast number of variations exist.It should also be appreciated that the exemplary embodiments are onlyexamples, and are not intended to limit the scope, applicability, orconfiguration of the invention in any way. For example, electrodesaccording to embodiments of the present invention, although illustratedin proximity to a distal end of a lead, may be located at a positionanywhere along a length of an implanted lead. Therefore, the foregoingdetailed description provides those skilled in the art with a convenientroad-map for implementing an exemplary embodiment of the invention. Itshould be understood that various changes may be made to exemplaryembodiments without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A medical system, comprising: an implantablecardiac pacing pulse generator; a first lead, comprising an elongatedlead body including a first elongated insulated conductor and a firstconnector formed at a proximal end thereof; the connector including afirst electrical contact electrically coupled at a first polarity to thepacing pulse generator; a second lead, comprising an elongated lead bodyincluding a second elongated insulated conductor and a second connectorformed at a proximal end thereof; the connector including a secondelectrical contact electrically coupled at a second polarity to thepacing pulse generator, the pacing pulse generator comprising means fordelivering pulses between the first and second electrical contacts; afirst pacing electrode joined to the first lead body and coupled to thefirst contact of the first connector via the first conductor, the firstelectrode adapted for intimate contact with tissue at a first site; asecond pacing electrode joined to the second lead body and coupled tothe second contact of the second connector via the second conductor, thesecond electrode adapted for location at a second site; and a porouslayer formed over the second electrode, allowing conduction therethroughwhile preventing contact between the second electrode and tissue inproximity to the second site.
 2. The medical system of claim 1, whereinthe second electrode includes an outer surface, the porous layerincludes an outer surface, and the second lead body includes an outersurface; the outer surface of the second electrode recessed from theouter surface of the second lead body and the outer surface of theporous layer isodiametric with the outer surface of the second leadbody.
 3. The medical system of claim 1, further comprising means topromote wetting of the porous layer.
 4. The medical system of claim 3,wherein the means to promote wetting comprises a wetting agent appliedto the porous layer.
 5. The medical system of claim 4, wherein thewetting agent comprises a surfactant.
 6. The medical system of claim 3,wherein the means to promote wetting comprises a surface treatment ofthe porous layer.
 7. The medical system of claim 1, wherein the porouslayer has a thickness between approximately 0.005 inch and approximately0.020 inch.
 8. The medical system of claim 1, wherein the porous layerincludes pores having sizes ranging, on average, between approximately0.4 micron and approximately 50 microns.
 9. The medical system of claim8, wherein the pores have sizes ranging, on average, betweenapproximately 0.4 micron and approximately 10 microns.
 10. The medicalsystem of claim 8, wherein the pores have sizes ranging, on average,between approximately 10 microns and approximately 20 microns.
 11. Themedical system of claim 8, wherein the pores have sizes ranging, onaverage, between approximately 20 microns and approximately 50 microns.